Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Pain is classified according to cause into nociceptive pain, neuropathic pain and psychogenic pain.
Neuropathic pain is pathological pain caused by peripheral or central nervous system dysfunction, more specifically, pain caused by e.g., direct damage and oppression of the nerve tissue despite of no nociceptive stimulus to a nociceptor. As the therapeutic agent for neuropathic pain, an anticonvulsant, an antidepressant, an anxiolytic drug or an antiepileptic drug (gabapentin, pregabalin or the like) is used.
In the meantime, compounds having the structure of a derivative analogous to our compounds as a partial structure are known in Japanese Patent No. 4563675. The possibility that the compounds may have drug efficacy to excessive overweight or obesity is suggested. International Publication WO 2006/137465 discloses that some nitrogen-containing heterocyclic derivatives exert an antagonistic action specific to one of NMDA receptors, i.e., NR1/NR2B receptor. International Publication WO 2013/147160 discloses that imidazole derivatives exert an analgesic action.
However, therapy with a conventional therapeutic agent for neuropathic pain is highly frequently associated with central nervous system adverse effects (e.g., dizziness, nausea or vomiting). Since it is difficult to administer such a therapeutic agent for a long term, development of a novel therapeutic agent for neuropathic pain has been desired.
Whether or not the substituted piperidines described in JP '675 have analgesic action including action against neuropathic pain is not known. Usefulness of the substituted piperidines described in JP '675 as a lead compound for an analgesic agent, particularly a therapeutic agent for neuropathic pain, has not yet been reported. It has been suggested that the nitrogen-containing heterocyclic derivatives described in WO '465 may have usefulness as an analgesic agent. It is disclosed that the imidazole derivatives described in WO '160 have an analgesic action.
It could therefore be helpful to provide a compound having a strong analgesic action for pain, in particular, neuropathic pain.